Google Summer of Code

Today, I’m delighted to announce something that I’ve been working on the past four months, which is now alive and doing exactly what it was meant for. A Standalone, turnkey logic analyzer based on the work I did for BeagleLogic. And is aptly titled – BeagleLogic Standalone.

BeagleLogic Standalone is a specialized version of the BeagleBone which is intended to be used a logic analyzer based on BeagleLogic.

Describing it as an equation, (BeagleLogic Standalone) > (BeagleBone) + (BeagleLogic). The greater than comes from the fact that the BeagleBone + BeagleLogic has up to 14 logic inputs, BeagleLogic Standalone has 16. It has 1000Mbps Ethernet, while the BeagleBone has just 100Mbps.

This logic analyzer has networking capabilities (10/100/1000Mbps Ethernet); it can be used to used to debug circuits remotely. And as it is a full-featured Linux computer, you can run the sigrok set of tools directly on the BeagleLogic Standalone board (they come preinstalled in the BeagleLogic system image), or on your host PC. It has 16 channels and can sample up to 1.5 seconds of data at the maximum sample rate, which is 100MSamples/sec (3 seconds of data if using only the first 8 channels).

If you are interested in purchasing one of these boards, please fill up this form, and I’ll keep you informed.

The rest of this post is dedicated to describing the story behind the creation of BeagleLogic Standalone.

A year and half ago I published a survey on the BeagleLogic wiki where prospective visitors and users were asked which feature they wanted to see in BeagleLogic the most. I compiled the raw responses so far, here they are:

It’s interesting that a majority of prospective users wanted to be able to do analog sampling with BeagleLogic.

And today, it becomes a reality for BeagleLogic users, thanks to the efforts of a team working at Google Research who wanted to use the BeagleBone for data acquisition and had been working independently on an ADC cape for the BeagleBone for a while. When we first got in touch, they already had a board fabricated and assembled and kindly agreed to send me a prototype to let me help BeagleLogic support the board. It excited me as not only would analog sampling support in BeagleLogic become a reality but also their project could benefit from the kernel infrastructure BeagleLogic has established to capture data using the PRUs on the BeagleBone and move it to userspace to be able to realize the full performance of the ADC which would be difficult to achieve through a libprussdrv based solution.

I am happy to tell you today about the PRUDAQ project (link to announcement on Google Open Source Blogs) – an ADC cape for the BeagleBone for doing high-speed analog data acquisition. At the heart of the board is an Analog Devices AD9201, a 10-bit ADC that can sample two channels up to 20MSPS.
The board has been designed by Jason Holt and his team at Google Research and the team at GroupGets as the manufacturing partner have been instrumental in getting the boards manufactured and ready for sale – the board itself is available at their store, and they also offer a bundle with pre-loaded SD card and other accessories.

GroupGets is a secure platform to create or join group buys for things that are out of reach for a single buyer like a high minimum order quantity for a specialty sensor. Their GetLab engineering team also creates custom hardware and software to enhance group buy targets or make them easier to use.

PRUDAQ with the BeagleLogic stack can be a great way for high performance analog data acquisition – the PRUs and the kernel driver can handle simultaneously both channels of the ADC at sample rates up to 19.9 MSPS 1, which corresponds to a data rate of 79.6 MB/s 2. The dual-PRU architecture of BeagleLogic that maintains a clear separation between the sampling operation itself (done by PRU1) and data transfer to memory (done by PRU0) means that on an application level, only the firmware on PRU1 needs to be changed in order to support PRUDAQ, no modification is necessary on the kernel driver side, raw analog data can be directly read through /dev/beaglelogic and this also opens the door to other ADC boards or sensors being able to make use of the high-speed capture framework provided by BeagleLogic by writing suitable firmware extensions.

There’s also an update to the BeagleLogic system image that also builds in PRUDAQ support, and this will be bundled by default in all new BeagleLogic image releases. It is immediately available for download from the images page on the BeagleLogic wiki. For those interested in the firmware, it’s present in the main repository now here. To learn more about the PRUDAQ project, their wiki is a good place to start.

The performance of PRUDAQ has been documented really well and it also highlights the bottlenecks in the later part of the document, which is also relevant to the performance of BeagleLogic.

I’d also take the opportunity to announce collaboration with GroupGets for the first production batch of BeagleLogic capes and a bundle offer – while the BeagleLogic capes aren’t available today, those who purchase the first batch of the PRUDAQ boards from GroupGets will get 10% off their purchase of a BeagleLogic cape as and when it is made available from GroupGets. The first batch buyers of the board will receive a discount code in their email when it’s ready.

Congratulations to the team at Google Research and GroupGets for a successful launch! I am also very grateful to the support provided to me by the Google Summer of Code program and the BeagleBoard.org Foundation in bringing this project to you over the summers 2 years ago, and looking forward to see the new applications that this will make possible.

Due to the nature of the assembly code that samples the PRU pins, it is not possible to sample both channels at 20MSa/s as the external clock needs to be tracked and at 20MSa/s, samples were found to be dropped. However it works OK at 19.9 MSPS with an external clock [tests done by Jason and his team]. It is however, possible to sample both channels simultaneously at the max possible 20MSPS if PRU1 is allowed to clock the ADC. This is a special case and the firmware to do so will be made available in the future. ↩

At demonstrations whenever I am asked a question about how BeagleLogic works, it takes time to be able to explain how a low cost SBC can actually sample at digital signals at 100 MHz, what makes the BeagleBone Black so special, why can’t this be done with something like a Pi without adding any extra hardware.

This blog post is an attempt to start from (almost) scratch and explain the nuts and bolts of the BeagleLogic assembly and document the design decisions made last year as a reference for future application scenarios. This should be the first in a series of posts.

The PRUs, or the “Programming Real-Time Units” on the AM3358 SoC on the BeagleBone Black are two 200 MHz microcontrollers that run side-by-side the 1 GHz ARM CPU. They can be started, stopped, reset, programmed via the CPU and also share the same bus so the PRUs can independent of the CPU access the core peripherals like GPIO, memory, ADC, DMA, … and also have a GPI/GPO subsystem of their own (“Enhanced GPIO subsytem”).

These PRUs can be programmed in C using the TI PRU C Compiler (recommended) or PASM (now considered obsolete but still works) or a GCC Compiler port (in progress) and of course, hand tuned assembly code one can write for the PRU as well.

At the heart of a logic analyzer…

… is a register that samples the input signal(s) at regular intervals (decided by the sample rate in case of a free running sampler) or at the edge of an external clock signal. These samples are then recorded into the sample buffer which can be then used to extract and analyze the captured digital signal.

Note that this is the simplest case, in real life scenarios there are often trigger conditions like ‘Hey, start recording when there is, say, “a falling edge on pin B when pin A is high”, or “after 5 rising edges on pin C when pin D is high and pin E is low”‘ and so on.

The core of BeagleLogic is the simplest possible implementation. It simply samples and records the inputs into a buffer at a sample rate that can be configured in integer divisions of 100 MHz i.e. 100 MHz (100 / 1), 50 MHz (100 / 2), 33.33 MHz (100 / 3) and so on.

Now let us look at the building blocks available in the PRUs of the BeagleBone Black that enable us to achieve this.

MOV, SBCO and LBCO Instructions

A quick primer on the 3 most important instructions for data transfer in PRU assembly. For a detailed overview refer to the PRU instruction set manual.

SBCO is used for moving data between a physical address and a register. This physical address can be within the PRU (e.g. PRU data RAM, shared RAM, power and control registers) or outside peripherals like the GPIO subsystem, the ADC subsystem, the EMIF subsystem (the DDR SDRAM controller), the GPMC. General usage:

SBCO &src, destination, offset, n

Moves n bytes from the src register into (*destination)+offset, this is indirect addressing. If n > 4, then the subsequent registers are accessed as well. Offset can be a register or an immediate value.

When using SBCO to write to the DDR memory, note that we must provide only physical memory addresses as there is no MMU involved when PRU accesses it. This is what most examples of the PRU that demonstrate shared memory access do.

LBCO loads data from an external address into a destination register. If n > 4, data gets loaded into the subsequent registers as well.

LBCO &dest, src, offset, n

Clock cycle counting

I’ve used the Cortex M3/4 microcontrollers and these cores have this nice register called DWT_CYCCNT which provides number of clock cycles the processor has executed code. So one can take the difference of DWT_CYCCNT register before and after a code block and get the number of cycles this code takes to execute. This allows cycle-accurate code profiling.

Lucky for us, the PRU has this neat feature as well, that I used for determining the number of processor cycles each instruction takes to execute. This is known as the CYCLE register1. But before we can use it, we’ll have to enable it by setting bit 3 in the CTRL register using this code snippet:

Now we can examine the contents of R1 and R2 to determine how many cycles it takes. You would also have to account for the “extra” clock cycles of the LBCO instruction.

I ran a lot of tests using this initially, here’s what I found:

every MOV operation is one cycle. In fact, any operation that does not access external memory or peripherals completes in a single cycle i.e. 5ns. This is by design.

LBCO and SBCO instructions with byte count 4 take 2 cycles. The way I hypothesize is that 1 cycle is spent to generate the address by adding the offset to it and then the actual data transfer operation takes 1 cycle per 32 bits transferred, thus O(n) time. Therefore the SBCO example in the previous section should take 9 cycles to complete (1+32/4), assuming there is no bus stall while writing the data to the memory.

We will use this information to help us with the timings needed for sampling.

Enhanced GPI/GPO feature

The PRU has an enhanced GPIO that operates at 200 MHz2 and it implements a “Direct Input” GPI mode. What it means that whatever be the pin value at the PRU inputs at the sampling instant will be captured whenever register R31 of the PRU is read. So, to sample first 8 bits of R31 into the register file, we can use the following PRU assembly code:(edited for more clarity, referring issue #9 on GitHub) The first line of the snippet below shows how one can sample the lowest 8 bits of R31 which is connected to the PRU input pins. The entire snippet shows how to make 5 samples of the input pins and store them into successive registers.

We can and pack up to 4 samples into a single 32-bit register using the PRU assembly instruction (Rn.bx refers to the x’th byte in the register – each register is 4 bytes).

You might ask, why sample to the registers, and why not store this data in the 8 KB SRAM, or the 12 KB shared RAM, or even the DDR RAM?

From the previous sections, we see that every SBCO would take 2 cycles but register access is just 1 cycle, so we can achieve higher sampling rates.

While accessing the DDR RAM there is a very low but finite probability of bus conflicts while data is being written, and having such an instruction within the real-time sampling loop has the potential to compromise the sampling operation. So we would like to separate both of them.

Right now, we’ve only stored data in the registers, and there isn’t enough space to store all samples in the registers. So we need to somehow get the data out of there.

The PRU0/1 Scratchpad and the XIN/XOUT instructions

​I remember initially reading this section in the PRU reference manual with skepticism and was disappointed to find scarce resources and/or example applications in the early phase of my GSoC period. But this turned out to be one of the important links in the puzzle.

​Apart from the 30 registers in the two PRUs (R30 and R31 are connected to the GPO / GPI respectively), there’s also 30×3 independent register banks available as a scratchpad; and this is connected to the registers on both the PRUs using a “broadside interface”. Broadside means that all 30 registers are connected, and all of them can be moved in parallel. This means that in a single clock cycle one can copy or swap all 30 registers of a PRU with one of these 3 banks.

This ability of moving bytes across the PRU barrier is crucial for BeagleLogic as it means that:

We can cleanly separate pin sampling (handled by PRU1) and data transfer (handled by PRU0).

Since pin sampling operates only on registers, it is effectively shielded from bus latencies.

Because register manipulation is cycle accurate we can design delay loops in PRU1 to give us a programmable sample rate, independent of PRU0 operation.

PRU0 can now directly push data into the 512 MB(!) of DDR memory directly giving BeagleLogic a buffer capacity so large at this price point. Note that due to packing the samples into 32 bit words the number of write transactions is cut down by a factor of 1/4th (8 bit samples) or half (16-bit samples) as compared to the sample rate, giving us cycles to spare. [The actual PRU firmware of BeagleLogic writes 32 bytes at a time into the DDR memory.]

Inter-PRU signaling

The final piece in the puzzle for a basic implementation is to have a way so that PRU1 can signal PRU0 that it has pushed data into Bank0 using XOUT, and that it can take data in using XIN and write it to the DDR memory.​ Interrupts. By configuring the mapping appropriately in the PRU interrupt controller (PINTC) one can send a signal to the other PRU by writing a value into the R31 register which triggers an interrupt (note we generally read from R31, not write). Similarly by waiting on bit 30 of R30 to be set, one can know from the other PRU when an interrupt has occurred. The interrupt configuration and mapping is in general handled by the library (libprussdrv) or the pru_remoteproc kernel driver, we just assume within the PRU that everything is already configured for us and is working.

​Putting it all together

Let us try writing a sketch of a very simple firmware which implements a basic form of a 8-bit logic analyzer. This should be helpful when developing for a similar application scenario​ and understanding the behind-the-hood working of BeagleLogic.

Observe how writing an infinite loop this way allows us to sample 4 bytes, move it into Bank0 and signal the PRU0 that data is ready. Also, the time gap between two samples is 2 clock cycles, so this infinite loop samples the pins at 100 MHz. See? How could one sample at, say, 50 MHz?

Next, let’s write the segment of code in PRU0 to receive this and write it into memory

Now observing how BeagleLogic does it – PRU1 Sampling code and PRU0 memory writing code should give an idea of the basic processes happening that make BeagleLogic possible. Of course, there is the overhead but it’s the same basic principle.

A similar post explaining the BeagleLogic kernel module is on it’s way soon.

A day before Christmas, I was greeted by a yellow DHL packet from Hong Kong. The panels had finally arrived after 8 days – I had chosen rush (48hrs turnaround) and express shipping, and it did work out well as I received them before Christmas. The boards turned out to be nice. Not dirty at all and at a price which was reasonable to me.

Assembly

Before assembly – the board with applied solder paste and a few parts

I reflowed the board with a hot air gun. Applied solder paste with a toothpick and placed the parts with the naked eye using a pair of good quality tweezers. The passives are all 0603 size (1.5mm x 0.76mm). Since I was yet to receive the batch of BSS138 FETs I ordered, I used a BC847 NPN transistor for the build instead. Reflowing left a couple of solder bridges on the 74LVCH16T245 (now onwards referred to as ‘the 245’) buffer chip (smallest pitched part on board – 0.5mm) which I removed using a solder wick. Then soldered the pin headers manually.

Cleaned up after soldering using nail polish remover. It’s terribly inefficient, and I plan to get a bottle of concentrated isopropyl alcohol (IPA) at some point of time. Here’s the finished result:

BeagleLogic – Components side

BeagleLogic – Top View

“Smoke Test”

A colloquial reference to the first power-on of the assembled circuit. I plugged the cape into the BeagleBone Black and powered on the assembly. The LEDs lit up and the board booted. No smoke or burning smell. Woohoo, test passed, or …

Errata

I then probed the BeagleBone P8 header pins which serve as inputs to BeagleLogic using an LED, and did the same with the input pin headers. The LED were brighter on the inputs to the ‘245 than the P8 pins, which was contrary to expectations. A quick check on the schematics and turns out that I wired the 245 to translate from rail B to rail A while connecting the inputs to rail A and the Bone pins to rail B. Whoops! However I quickly resolved the problem by lifting pads 1 and 24 (DIR1 and DIR2) off the PCB (which was simple as those were on the ends) and soldered a bit of magnet wire through and to one of the pads of C1 going to +3.3V. Neatly done, and a lesson not to design and send off PCBs to a fab while pulling an all-nighter 😛 .

Testing

Once this was done all was back on track and the circuit worked as expected. The cape as designed did not interfere with the boot-up process due to the action of the transistor pull-down that ensured that the buffer did not drive the P8 pins until SYS_RESETn signal was high.

I tested it with SPI signals upto 24 MHz using the BeagleBoard itself and results are good at a 100Msps sample rate. I could also subject the input pins to 5V freely, without fear that I would burn the board. That’s exactly what the cape is meant for, and I’m happy with the results.

I am planning to give away the surplus boards from the first batch so anyone interested in testing it out can get in touch and depending upon where you are, I may be able to send you one of the surplus unpopulated boards (no parts included) which can be assembled yourself (1 IC, 4 passives, 1 transistor and pin headers). The Cape EEPROM section can be left unpopulated without affecting the functionality.

The design files are now available here. I made some changes to the design after fixing the errata, so the cape version is bumped to 1.1 .

Suggestions and feedback on the cape are welcome. The design is Open Hardware, so you have the freedom to use it and improve it as you like. Let me know if there’s anything that could be added in the cape as there is plenty of board real estate.

After coding up the BeagleLogic project, I thought that it would be great to have an add-on cape for the project that provides buffering and also makes the inputs of the BeagleBone Black tolerant to TTL logic voltage levels (up to 5.5V) allowing BeagleLogic to debug external projects with ease. Hence introducing the BeagleLogic cape, the 3D render of which you can see above. The design is done in KiCad.

Design & Layout

The cape design is simple enough to just have a single layer layout, as you can see in the render above the top layer is entirely a ground plane but for a single trace. Since the top isn’t much populated I added useful information on the top silkscreen including indexing the pin headers on the Bone on both sides.

The logic channels are accessed via 2×14 right angled pin headers. The upper row of headers are the actual logic channels while the bottom row is all GND pins. The pin headers are arranged in a MSB-to-LSB fashion. This means that the rightmost pin when viewed from the top is raw bit 0 of the captured logic samples. Note that sigrok will use the names of the actual Bone pins so bit 0 (Channel 1) is to be identified as P8_45, bit 1 (Ch2) is P8_46 and so on. The numbering is a little non-obvious but it’s because that’s the way the pins are arranged on the BeagleBone GPIO header. But don’t worry as the cape lists the pin ID of each logic channel so you don’t have to look it up in the pin diagrams.

One important point here. Only the first 12 channels can be used by default. To use the last two channels, you must disable eMMC first and solder 0R resistors or bridge the two resistors R8 and R9 on the bottom side to enable them. Otherwise the buffer will drive those two pins and you will damage the eMMC of the board and also void the warranty.

Here’s a shot of the schematic (click to enlarge). This is for reference only with respect to the current board and the released schematic may or may not be the same

The active buffer is a TI 74LVCH16T245 or equivalent. The buffer is powered from the VDD_3V3B power rail. The OE pin initially pulled is driven using an arrangment of a BSS138 N-MOSFET whose gate is connected to SYS_RESETn of the Bone. This should ensure that the logic input pins, which are also the system boot pins, are not driven by the buffer until the startup has completed.

This version of the design has a 0R resistor through which the VDD_3V3B powers the VDDA side rail of the 16245. If you remove the short and connect it to a 1.8V supply it should become compatible with 1.8V logic levels. I am however thinking of a better solution to the problem and should address this in the next released design.

There’s the officially required cape EEPROM on the bottom side as well, I presume this could be rendered redundant as the community moves towards the Universal Cape concept. But the footprints are there, just in case.

Manufacturing

The first prototype cape has been manufactured by DirtyPCBs.com as a 2-layer Black 10x10cm protopack. It has been shipped as of the time of writing and should reach me next week. I ordered the boards as a Rush order (48h turnaround time) and got it shipped via DHL so that I could have the boards in hand before Christmas rush. I would be using their services further if the boards work out well, looking forward to receive them!

Since I had left space on the panel, and there’s free panelizing so I managed to squeeze some more of my designs into the panel and make the best use of the available real estate. I would write more about those in the coming posts.

So that’s pretty much it. Design suggestions are welcome, and I’ll see if they can be accomodated in the subsequent hardware revisions. Once I test and it all works, the design files will be made available as I have written above.

BeagleLogic was born out of a project idea using the BeagleBone Black, and I am thankful to Jason Kridner and the BeagleBoard.org community for accepting it as a mentored project under the Google Summer of Code 2014. As the GSoC program officially ended last week, here is a video report highlighting what’s been accomplished so far, over a period of 3 months from 19th May to 18th August.

It was an awesome experience this summer, interacting with the community and developing the codebase for a project that showcases the capability of the two little yet powerful integrated Programmable Real-Time Units (PRUs) on the AM335x SoC that powers the BeagleBone Black to realize a 14-channel, 100Msps, 360 MB buffer Logic Analyzer which by far, offers the best value for a 55$ board which you can use not only to develop your embedded projects but also to debug them on-site without any extra hardware requirement. All the magic happens inside the firmware loaded into the PRUs and the kernel driver which manages sharing the system memory with them.

This project also saw me doing many things for the first time. It was the first time I compiled a Linux kernel and wrote myself a kernel module that is now a part of the BeagleBone community kernel, and I look forward to getting it merged upstream. The Web Client for BeagleLogic (link) is my first web application using HTML5 and Bootstrap for a lightweight web client for BeagleLogic. The backend I’ve written for the web client is my first Node.JS based application. I found these frameworks quite interesting and hope to work on projects in these areas in the future.

Google Summer of Code has been the best thing to happen to me so far, and I’ll be looking forward to it in the years to come.

Have fun with BeagleLogic. Also, do write back to me with how BeagleLogic helped you debug helped you debug your circuit and learn more about logic protocols!

This week I spent some time familiarizing myself with the libsigrok components, and developed bindings for BeagleLogic into libsigrok. The development work walked me through non-blocking I/O, which I implemented into the BeagleLogic kernel module only to realize that it had a major bug that causes the client to sleep uninterrupted and froze my BeagleBone Black everytime. The bug got identified and fixed and after that non-blocking I/O worked as expected.

Memory Mapped Kernel Buffers
This was another optimization I made use of in the sigrok bindings. The sigrok library internally keeps logic data in packets and it is exchanged with the host application via pointers. I exploited this good opportunity to use the mmap() functionality I designed into the kernel driver, so the sigrok implmentation just passes a mmap()’ed pointer to the host application instead of read()-ing the data.

However the currently being read buffer still needs to be updated for poll() to work properly. For this reason I implemented dummy reads. When a NULL pointer is passed as the buffer to read(), it does not read any data from the buffer, and just updates the read state within the drivers so that poll() works fine.

The flow of data is like this:

libsigrok bindings signal the BeagleLogic module to start the sampling operation, and starts polling on the /dev/beaglelogic node using a GPollFD

Once the PRU fills up a data buffer, it sends an IRQ that wakes up the the polling thread and the beaglelogic_receive_data() callback in protocol.c executes

The callback sends a packet with the pointer to that segment of data required, and adjusts the internal offsets for the next callback

The process goes on till the sample limit has been reached. BeagleLogic also supports streaming but…

sigrok-cli on the BeagleBone Black
To test the results of the bindings, I ran a sample operation on BeagleLogic using the sigrok bindings and it worked fine. I was able to decode a wave file from my trusty audio player hardware 🙂 to wav right there on the BeagleBone Black using the article on the sigrok blog as a reference. I got 6 seconds of a wave file directly from the dump. This took ~750 seconds to process from RAM on the BeagleBone Black.

The first week was spent well in prototyping the PRU firmware. Problems with the memory management [Maximum of 8 MB of memory] and dismal memory copy speeds [10-20 MB/s] with UIO and libprussdrv prompted me to escalate the back-end to the remoteproc kernel driver. It took me a while before I started off based on the WS28xx lighting firmware example to build up the necessary kernel driver infrastructure for BeagleLogic, and it’s now close to the first set of stress tests and actual data capture for the PRU.

Going into the kernel driver, here is a summary of what’s coming up in the next update [the experiment is with dummy data, the PRU’s aren’t engaged] :

The first week of the Summer of code began with the build of the core PRU firmware for data capture. I coded up firmware for both the Programmable Real-Time units operating in tandem to sample the pins and transfer it to the DDR memory directly. The code can be viewed at these links:PRU0 assembly codePRU1 assembly code

It makes good use of the XIN/XOUT broadside interface available for inter-PRU communication allowing movement a chunk of sampled data from PRU1 to PRU0 [currently 8 registers = 16 samples = 32 bytes] in one clock cycle. PRU0 then writes the data into the DDR memory in bursts of 32 bytes. Inter-PRU signalling is achieved through interrupts.

For buffer overflow/underflow detection, there is a global byte counter running in PRU0, which is moved alongwith the logic data “for free” via XOUT. PRU0 compares the received value of the counter with the value received from the previous interrupt, and if they differ by more than 32, then there has been an overflow. Also, when the ARM core is signalled for data, an interrupt counter is incremented. The counter is compared to its previous value, and the delta here again enables us to determine if an underflow has occurred.

This approach works fine for one-shot sampling, and I have been able to achieve all the way up to 4 MSamples of 12 pins running at 100 MHz [40ms], although the limitation on the maximum sample rate is likely to be the hardware of the BeagleBone Black in this case, remember there’s a 47pF capacitive load on the HDMI shared pins, and it is yet to be tested with actual hardware.

Currently there’s only 8 MB of memory shared with the PRU for storing samples, there’s an issue with the UIO kernel driver that prevents reserving more memory. The UIO driver will not be fixed, rather the issue will be addressed in the remoteproc interface of the PRU with the kernel. Until then, there is a workaround by adding mem=448m to the boot command line in uEnv.txt, to reserve the upper 64 MB of the memory for the PRU.

Reducing the sample rate is just inserting more NOPs into the sample loop to adjust the cycles. However, availability of more room between two sampling instructions turn out to be potential cycles for performing RLE (which will be implemented very soon), which seems achievable at 50 MHz.

Overwriting previous chunks of data works fine as well, stress tested upto overwrite 500x. See here for an example run.

The amount of samples collected is still limited by only the amount of memory available. The PRUs are quite fast and capable of sample rates like 100 MHz. The only bottleneck is to hold the data and process it before it runs out, and this is the target for the coming week.

The current test code will be adapted to the sigrok bindings once it is in good shape. The current stable code is available at the repo here, and the latest development is in the “prutest” branch here

This week I spent time understanding the remoteproc implementation and UIO implementations of the kernel drivers of the PRU in the kernel source tree.

One of the important decisions this week was the confirmation of UIO implementation for the PRU drivers. Although a remoteproc implementation is a better approach, the initial implementation of the core would be in UIO, and as the remoteproc infrastructure improves, BeagleLogic will eventually migrate. Since this would be a core change, it would have minimal impact on the functionality.